Cardiac Resynchronization Therapy (CRT), delivered by implanted pacemakers and intracardiac cardioverter defibrillators, is currently an established therapy for patients with congestive systolic heart failure and intraventricular electrical or mechanical conduction delays, as described, for example, in Clinical Cardiac Pacing and Defibrillation, 2nd edition, Ellenbogen, Kay, Wilkoff, 2000. Cardiac output depends strongly on the left heart contraction being in synchrony with the right atrium and ventricle of the heart (see U.S. Pat. No. 6,223,079). Congestive heart failure is defined generally as the inability of the heart to deliver enough blood to meet the metabolic demand. Often, congestive heart failure is caused by one or more electrical conduction defects. The overall result is a reduced blood stroke volume from the left side of the heart. For congestive heart failure patients, CRT is an effective treatment employing a permanent pacemaker with electrodes in three chambers that re-synchronize the pacing of the atrium and the left and right ventricles [“Device Therapy for Congestive Heart Failure”, K. Ellenbogen et al, Elsevier Inc. (USA), 2004]. The resynchronization task demands exact pacing management of the heart chambers such that the overall stroke volume is maximized for a given heart rate. Optimal timing of activation of the atrium and the right and left ventricles is one of the key factors influencing cardiac output, where the main intent is to cause the left ventricle to contract in synchrony with the right ventricle. The timing parameters that are programmed in a CRT device that determines the pacing intervals are atrioventricular (AV) delay and interventricular (VV) interval. AV delay is the delay in cardiac pulse moving from the atria to the right ventricle; and VV interval is the time interval between right and left ventricle stimulations. The VV interval can be either negative or positive. When the VV interval is negative, the left ventricle is stimulated before the right ventricle; and when the VV interval is positive, the left ventricle is stimulated after the right ventricle. When monitoring cardiac pacing, two major parameters are registered—PRV and PLV. PRV is the right ventricle pacing interval, or in other words, the time interval between sensed atrial stimulation and sensed right ventricle stimulation. Thus PRV is equal to AV delay. PLV is the left ventricle pacing interval, or the time interval between sensed atrial stimulation and sensed left ventricle stimulation. Hence, PLV equals AV delay plus VV interval.
The re-synchronization task is patient and activity dependent, in that for each patient the best combination of pacing time intervals that restores synchrony is changed during normal daily activities of the patient.
The reasons that the currently available CRT devices cannot achieve optimal delivery of CRT are as follows:
1. Programming and troubleshooting the CRT device—currently, optimizing the CRT device using echocardiography is expensive, time consuming and operator dependent. The clinician is required to optimize both the AV delay and VV interval in order to achieve resynchronization of heart chamber contractions.
2. Consistent Delivery of CRT—There are several reasons why CRT is not delivered consistently, and at times not delivered at all for hours. Two reasons for this are failure to optimize the AV delay and the low value of the programmed maximal tracking rate.
3. Follow ups—The clinician must perform the complex task of optimization and programming of the CRT device, first during implantation and then at each follow-up.
4. CRT non-responders—a significant number of patients, typically about 30%, do not respond to CRT after implantation. The development of good markers that will enable identification of responders to CRT is a major issue due to the complexity of the instrumentation, the need for device implantation, and the medical costs associated with the treatment (David A. Kaas, “Ventricular Resyncronization: Patophysiology and Identification of Responders”, Reviews in Cardiovascular Medicine, Vol. 4, Suppl. 2, 2003).
In this respect, Hayes et al. In “Resynchronization and Defibrillation for Heart Failure, A Practical Approach”, Blackwell Publishing, 2004, suggest that optimal programming of the CRT device may turn “non responders” into “responders” and “responders” into better “responders”.